1. Field of the Invention
The present invention relates to a high density package substrate and method for fabricating the same, and in particular relates to a method for forming a high density package substrate by laser drilling.
2. Description of the Related Art
Along with technological development, electrical devices are becoming lighter, thinner, shorter and smaller. As such, printed circuit boards in electrical devices are also being required to become lighter, thinner, shorter and smaller. Thus, high density interconnection (HDI) technology, allowing for high density interconnection, fine spacers and miniaturization, is being used in printed circuit boards to provide increased functions in a limited space and further decrease fabrication process costs. For HDI technology, via-on-pad, with a plated through hole formed on a pad, is utilized. Additionally, because a conventional mechanical drill destroys copper foils of a double-sided copper clad laminate, laser drills are used to more accurately control the location and size of holes for the via-on-pad.
There are three main methods for forming the plated through hole of the double-sided copper clad laminate. The first method is a full-subtractive process, the second method is a semi-subtractive process and the third method is a modified semi-additive process.
The thickness of the double-sided copper clad laminate copper foil used in the full-subtractive process is generally about 12 μm (or 18 μm). When the through hole is formed by laser drilling, the thickness of the lower copper foil is at least larger than 12 μm, so that a punch through problem caused by a laser drill is prevented. However, on the other hand, the thick copper foil increases etching time. Therefore, increasing etching time worsens an undercut phenomenon of the printed circuit board and negatively affects the forming of fine lines.
The thickness of the double-sided copper clad laminate copper foil used in the semi-subtractive process and the modified semi-additive process is generally only about 4 μm. The advantage of both processes is that the thinner copper foil improves the undercut phenomenon of the printed circuit board and improves forming of fine lines. However, the ultra-thin copper foil causes other problems, such as complicating the laser drilling process, increasing costs and increased probability of punch through.
Accordingly, there is a need to develop a process, which not only prevents the punch through caused by laser drilling, but also allows the double-sided copper clad laminate to have fine line capability.